1. Field of the Invention
The present invention relates to a solid-state imaging device and an electronic camera equipped with the solid-state imaging device.
2. Description of Related Art
Today, electronic cameras such as video cameras and electronic still cameras with an AF (autofocus) function are widely used by consumers. Such a camera typically includes a solid-state image sensor which may be, for instance, a CCD sensor or a CMOS sensor. The solid-state image sensor is constituted with a plurality of pixels, each equipped with a photoelectric conversion unit that generates a signal charge in correspondence to the amount of incoming light, arrayed in a two-dimensional matrix pattern.
Focus detection is executed in such an electronic camera in the related art through the contrast method, which enables AF based upon image signals provided from the solid-state image sensor. In the contrast method, the contrast is determined based upon image signals obtained through small adjustments of the focal length of the lens and the focus match is achieved by driving the lens to the position at which the contrast is at the highest level. This means that the focus match cannot be achieved quickly through the contrast method.
Japanese Laid Open Patent Publication No. 2006-261929 discloses an electronic camera adopting the contrast method described above. The camera includes a display unit disposed at the rear surface thereof or the like and constituted with a liquid crystal panel or the like, at which a live view image is brought up on display. The live view image, generated based upon the image signals from the solid-state image sensor, helps the user easily verify the photographing conditions including the subject framing state prior to the primary photographing operation.
The camera reads image signals from a CCD color area sensor by switching to a live view read mode and a summing read mode in correspondence to alternate frames when the brightness of the subject is low. At the display unit of the camera, an image is brought up on display based upon live view image signals read out while the live view read mode is on. The AF control unit of the camera executes AF control through the contrast method based upon brightness sum image signals read out while the summing read mode is on. In the live view read mode, live view image signals are generated based upon electrical charges read out through a discriminative read of a plurality of photoelectric conversion elements arrayed along the vertical direction at the CCD color area sensor by reading out electrical charges over intervals corresponding to a predetermined number of photoelectric conversion elements. In the summing read mode, brightness sum image signals are each generated by reading out the sum of electrical charges generated via a plurality of photoelectric conversion elements arrayed successively along the vertical direction or the horizontal direction at the CCD color area sensor.
Accordingly, when the brightness of the subject is lower, the AF control is executed in the electronic camera described above through the contrast method based upon the brightness sum image signals assuring a higher output level at a given level of ambient light compared to the output obtained through a regular read operation. As a result, stable AF control is assured through the contrast method. In addition, when the brightness of the subject is lower, an image is displayed at the display unit based upon the live view image signals. The subject color information can thus be obtained from the image on display.
The electronic camera switches between the live view read mode and the summing read mode only when the brightness of the subject is low and the switchover does not take place under normal circumstances in which the brightness of the subject is relatively high.
A solid-state image sensor assuming a structure that allows it to also function as a focus detection element adopting the split-pupil phase difference focus detection method has been proposed in recent years (see Japanese Laid Open Patent Publication No. 2003-244712). The split-pupil phase difference method differs from the contrast method in that it eliminates the trial and error approach with regard to the lens position, making it possible to achieve the focus match quickly.
The solid-state image sensor disclosed in Japanese Laid Open Patent Publication No. 2003-244712 includes a plurality of focus detection pixels (may also be referred to as “AF pixels”) via which focus detection signals to be used to detect the focusing condition through the split-pupil phase difference method are generated, disposed therein in addition to imaging pixels that output imaging signals used to express the subject image. The AF pixels in the solid-state image sensor each include a photoelectric conversion unit split into two separate portions. Over the photoelectric conversion units, micro lenses are disposed each in correspondence to one of the pixels. The two split photoelectric conversion units are disposed at a position at which the exit pupil of the imaging lens is image-formed by the micro lens, i.e., at a position substantially conjugate with the position of the exit pupil of the imaging lens relative to the micro lens. This means that since the distance between the exit pupil of the imaging lens and the micro lens is sufficiently long in relation to the size of the micro lens, the two split photoelectric conversion units are disposed at a position substantially matching the focusing position of the micro lens. At each AF pixel assuming the positional relationship described above, one of the two split photoelectric conversion units selectively receives light flux departing an area ranging over part of the exit pupil of the imaging lens and offset along a specific direction relative to the center of the exit pupil and the light flux thus received undergoes photoelectric conversion at the particular photoelectric conversion unit. The other photoelectric conversion unit at the AF pixel selectively receives a light flux departing the area ranging over part of the exit pupil of the imaging lens and offset along the opposite direction relative to the center of the exit pupil and the light flux thus received undergoes photoelectric conversion at the other photoelectric conversion unit.
An object of the present invention is to provide an electronic camera that includes a solid-state image sensor assuming a structure that allows it to also function as a focus detection element adopting the split-pupil phase difference method, capable of executing both AF control and AE control at high speed in a specific operation mode, and a solid-state imaging device that may be used in such an electronic camera.
Another object of the present invention is to provide an electronic camera capable of displaying a live view image of a subject present in a relatively short range and moving at a relatively high speed in a focus matched state on the subject and an optimal exposure state and also capable of executing AF control and AE control quickly in the particular operation mode and a solid-state imaging device that may be used in this electronic camera.